(1) Field of the Invention
The present invention relates to optical retardation means and more particularly to thin film retarders for improving the viewing angle and brightness of liquid crystal displays.
(2) Description of Related Art
Tremendous advances in liquid crystal display technology has created advanced display devices that are brighter, lighter, and capable of displaying information with high resolution. Due to the lightweight, smaller form factor and low power requirements of liquid crystal displays, they offer several advantages over alternative displays such as cathode ray tube displays. However, liquid crystal displays suffer from poor contrast and/or chromaticity at wide viewing angles such that viewing the display is often difficult in many viewing environments.
The primary factor limiting the contrast ratio achievable in a liquid crystal display arises from light that leaks through portions of the display that are supposed to be in the dark state. The contrast ratio of a liquid crystal also depends on the viewing angle of the observer. As is well known, the contrast ratio in a typical liquid crystal display is a maximum only within a narrow viewing angle centered about the normal incidence and drops off as the viewing angle increases. This viewing angle related decrease in the contrast ratio is caused by light leaking through dark state pixel elements at large viewing angles. Large viewing angles may also cause color shifts in color displays.
To improve both the contrast and viewing angle, optical compensator layers may be included in the liquid crystal display. The need for such compensators increases for supertwisted nematic (STN) liquid crystal displays where it is common practice to include a second STN display panel having an opposite twist to compensate for the birefringence of the first panel. Since it is expensive to provide a second liquid crystal display panel solely as a compensator, many displays include optical compensators such as phase retardation films. The use of such retardation films have been disclosed in U.S. Pat. Nos. 5,196,953 (issued to Yeh et al.), 5,504,603 (issued to Winker et al.), 5,557,434 (issued to Winker et al.), 5,589,963 (issued to Gunning, III et al.), 5,619,352 (issued to Koch et al.), 5,612,801 (issued to Winker) and 5,638,197 (issued to Gunning, III et al.). A liquid crystal layer is sandwiched between a pair of polarizer layers with one polarizer having an absorbing layer perpendicular to the absorbing axis of the other. With the addition of one or more compensator layers, the contrast ratio is improved over a wide range of viewing angles by careful orientation of the optic axis of the compensator. The combined retardation effect of the compensator cancels the phase retardation inherent in liquid crystal displays. All of the above referenced patents are either commonly assigned with this application or assigned to the assignee's parent corporation, Rockwell International Corporation.
One type of compensator included in the compensator layers discussed in these patents is known as A-plate compensator. An A-plate is a birefringent layer with its extraordinary axis (i.e. its c-axis) oriented parallel to the surface of the layer and its a-axis oriented normal to the surface (parallel to the direction of normally incident light). Prior art A-plates may be fabricated by the use of uniaxially stretched polymer films, such as polyvinyl alcohol or other suitable oriented organic birefringent material.
Unfortunately, the quality and uniformity of the optical properties of such prior art A-plates are generally poor. Further, the A-plate must be held under tension and the material must be relatively thick to obtain sufficient anisotropy. Further still, lamination of such A-plates with other retardation films often results in stress-induced birefringence when exposed to elevated temperatures.
Recently, it has been reported that retardation films may be obtained from ultraviolet (UV) curable liquid crystals. See, for example, H. Hasebe, K. Takeuchi and H. Takatsu, J.SID, 3/3, 139 (1995). As shown in FIG. 5, two substrates are provided so that traditional alignment techniques may be used to eliminate any splay in the crystal structure that may otherwise be introduced by the intrinsic tilt of the liquid crystal at the liquid crystal/air interface. However, since polyimide alignment material is very expensive, it is undesirable to apply alignment layers to two substrates. It is also undesirable to incur the expense associated with aligning and spacing the two substrates if one or both of the substrates are to be discarded after photopolymerization of the UV-curable liquid crystal. It is further still undesirable to rely on spaced parallel substrates to establish the orientation of the liquid crystal prior to photopolymerization since there is a practical limitation on the size and uniformity achievable with spaced substrates.
Clearly, it is desirable to minimize the number of substrates required to orient the UV-curable liquid crystal material. As noted in the above-referenced U.S. Pat. No. 5,619,352, polymerizable liquid crystals may be used in a process that coats a single substrate surface to create thin film compensators. This process eliminates the need to provide a second substrate having an alignment layer for aligning the liquid crystal. However, most polymerizable liquid crystals have an intrinsic non-zero tilt angle at the polymerizable liquid crystal/air interface that makes it difficult to achieve the planar orientation of the liquid crystal needed for A-plate compensators. Accordingly, an efficient and economical method is needed for achieving a tilt angle at the nematic /air interface necessary that is lower than the intrinsic tilt angle of the polymerizable liquid crystal.